Do parallel conductors have to be the same length?

Yes. NEC 310.10(G)(1) requires all conductors of each parallel set to be the same length, along with the same material, circular mil area, insulation type, and termination method.

What is the smallest wire that can be paralleled?

NEC 310.10(G) permits paralleling at 1/0 AWG and larger, with narrow exceptions for control power and certain frequency or capacity applications.

Does each parallel raceway need its own ground?

Yes. NEC 250.122(F) requires a full size equipment grounding conductor in each parallel raceway, each sized to the full overcurrent device rating rather than divided among the raceways.

When running parallel conductors, do they need to be the same length?

Same length is not a suggestion. Section 310.10(G)(1) of the NEC 2023 lists five conditions that every conductor of a parallel set has to meet, and the first one is that they be the same length. The others are the same conductor material, the same size in circular mil area, the same insulation type, and termination in the same manner.

Why the code is strict about it

Parallel conductors share current in inverse proportion to their impedance. A run of 600 kcmil copper has roughly 0.021 ohms of resistance per 1000 feet. If one leg of a parallel set is 10 percent longer than the other, that leg carries meaningfully less current and its partner carries more. The overloaded conductor runs hot while the breaker, which only sees the total, never trips. That is the failure mode the rule exists to prevent, and it shows up in the field as one discolored lug in a set of four.

What same length actually means

The code gives no tolerance figure, which trips people up. Industry practice is to cut every conductor of a set to a single measured length from one reel run and keep the differences within a few inches on a long pull. The requirement applies within a phase, so all the A phase conductors must match each other. The A phase set does not have to be the same length as the neutral set, though in practice everything is pulled together.

Handling a tight space

The usual fix when one path is shorter is to add slack to the short leg rather than shorten the long one. Coil the extra in the gutter or wireway, keeping bending radius under Chapter 9 Table 2 and box sizing under 314.28. Do not attempt to compensate by using a larger conductor on the long leg, because 310.10(G)(1) requires identical circular mil area.

The other parallel rules

Section 310.10(G) permits paralleling only at 1/0 AWG and larger, with limited exceptions in 310.10(G)(1) Exception for control power, frequency, and similar cases. Section 310.10(G)(2) requires the raceways or cables themselves to have the same characteristics, so do not run one leg in steel EMT and its partner in PVC. Section 300.3(B) requires all conductors of the same circuit, including the neutral and equipment grounding conductor, to be in the same raceway. Section 250.122(F) requires a full size equipment grounding conductor in each parallel raceway, sized to the full overcurrent device, not divided across the sets. Section 300.20(A) requires all phases in each ferrous raceway to prevent inductive heating, which is why a single phase per steel pipe is a violation.

Field takeaway

Cut every conductor in a set to one length, keep the material, size, insulation and terminations identical, put all phases plus a full size ground in each pipe, and coil the slack instead of trimming.